Wind Load A-frame Residential Structure

[WBell]

I am developing the wind loads on A-frame residential structure, dimensions 23'x25'x20' (Length x Width x Height). The project is located in North Carolina and the reference standard for wind load is ASCE 7-10. Although there is no a specific mention of A-frame structures in the standard, it appears there are at least 2 choices for determining the wind loads for the Main Wind Force Resisting System (MWFRS), which are:
i. Chapter 27, Part 2: Enclosed Simple Diaphragm Buildings With h <= 160 ft. The User Note states "Part 2 of Chapter 27 is a simplified method for determining the wind pressures for the MWFRS of enclosed, simple diaphragm buildings whose height h is <= 160 ft. The wind pressures are obtained directly from a table. The building may be of any general plan shape and roof geometry that matches the specified figures."
ii. Chapter 28, Part 1: Enclosed and Partially Enclosed Low-Rise Buildings. The User Note states "Use Part 1 of Chapter 28 to determine the wind pressure on the MWFRS of enclosed, partially enclosed or open low-rise buildings having a flat, gable or hip roof."

The A-frame structure is essentially a gable roof with a roof angle (theta) of 58 degrees (from horizontal) mounted on a very low wall/curb. I plan on mounting the roof on a knee-wall around the perimeter of a slab-on-grade foundation to keep the edge of the roof at least 18-24 inches above finish grade.

I would appreciate comments on whether or not it is appropriate to consider the A-frame structure as a "simple diaphragm building", which would lead me to use Chapter 27, Part 2 to obtain the wind loads on the roof. I will include some wind load on the knee-wall, but the majority of the surface area would be the gable roof with a steep angle. Chapter 28, Part 1 does not mention the necessity of having a simple diaphragm and would seem to be a method that would not be questioned in terms of applicability, even with the low wall height.

Thank you for any responses or comments.

 

[phamENG]

No, I would not consider it a simple diaphragm and I would not use chapter 28 part 1, either. I'd use Chapter 27, Part 1.

I had one of these come up recently, but the project died before it got off the ground. While walking through the procedure I was going to use, I realized that the actual behavior of one of these is not going to be a typical wall/diaphragm situation. With these sorts of angles, frame action will dominate over a more "typical" diaphragm and shear wall arrangement. That's one of the reasons you can get these with nothing but a big triangle of glass at each end.

 

[SWComposites]

When my parents A-frame cabin was built in MI in 1970 I doubt there was any engineering. Pairs of 2x12s for the frame “rafters” with 2x14s for second floor joists between each frame pair. Its quite stout and rigid, plus snow slides right off.

 

[gte447f]

I agree with phamENG's comments. I would add that a kneewall at the perimeter sounds questionable to me, unless you have a floor diaphragm at that level to brace the top of the kneewall.